Tax Yes or No in the total price

So let's say you have a field called SubTotal, which is just a straight-forward sum of all of your fields. Now, create a read-only text field called SubTotalTax (for example).
Now let's also say you have a (hidden) text field called Tax which has the value 0.19. We'll use that one for the tax calculation instead of hard-coding the value into the script. That way you could easily adjust it in the future, if necessary. And then there's a checkbox called UseTax, which determines whether or not the tax should be calculated.
Now use the following code as the custom calculation script of SubTotalTax:

var useTax = (getField("UseTax").value != "Off");

if (useTax) {
event.value = getField("SubTotal").value * getField("Tax").value;
} else {
event.value = 0;
}

Your Total field can simply be the sum of SubTotal and SubTotalTax. The only thing you need to make sure is that the calculation order is correct, ie that Total is calculated last, SubTotalTax before it, and SubTotal before that.